In language class, the girl to boy ratio is $10$ to $7$. If there are $30$ girls, how many boys are there?
A ratio of $10$ girls to $7$ boys means that there is a group of $10$ girls for every group of $7$ boys. If there are $30$ girls, then there are $3$ groups of $10$ girls each. Because we know the ratio is $10:7$ , there must be $3$ groups of $7$ boys each. There is a total of $21$ boys in language class.